1. Cell signaling Cells do not work in isolation but continually ‘talk’ to each other by sending and receiving chemical signals to each other. This process is known as cell signaling Cell signaling has a number of important steps  A signaling cell produces a signal molecule  The signal molecule is recognised by a target cell by means of a receptor protein  The receptor protein performs the first step in a series of transduction processes by converting the incoming extra-cellular signal to an intracellular signal that directs the cells behaviour

2. Principals Of Cellular Signalling Extracellular signal molecule A - IN Intracellular signal molecule B - OUT Change of molecule from A-B is an example of SIGNAL TRANSDUCTION Receptor protein ‘recognises’ signal molecule * * - this receptor protein may be on cell surface or inside cell

3. Signal molecules may be hydrophilic in nature ( e.g peptide hormones and neurotransmitters) or hydrophobic in nature (e.g. steroid hormones) Hydrophilic and hydrophobic signal molecules cross cell membranes by two different routes.

4. Hydrophobic signal molecules include steroid hormones such as cortisol, oestrogen (estradiol), progesterone and testosterone and thyroid hormones such as thyroxine. Steroid hormones diffuse directly through the cell membrane and bind to an inactive intracellular receptor protein known as a gene regulatory protein located in the cytosol or in the nucleus. On binding the intracellular receptor becomes active allowing it to bind to the equivalent regulatory sequence in the DNA Hydrophobic signal molecules

5. Cortisol Plasma membrane Intracellular Gene Regulatory Protein Intracellular gne regulatory protein is activated by slight change in shape (not shown) Activated Gene regulatory protein- steroid complex moves into nucleus Nuclear membrane DNA Activated receptor- steroid complex binds to the regulatory region of the target gene and activates transcription Activated target gene TRANSCRIPTION RNA The Effect Of Cortisol On Target Cells